Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DESCRIPTION
"PLANT AND METHOD FOR VACUUM DEGASSING LIQUID STEEL"
Field of application
[0001]The present invention relates to a plant and method
for vacuum degassing liquid steel.
[0002] The plant and the method according to the invention
can be used for vacuum degassing with either the VD
(Vacuum Degassing) technique or the VOD (Vacuum Oxygen
Decarburisation). technique and for all applications where
a vacuum treatment of liquid steel is required.
State of the art
[0003]The vacuum degassing process(called for simplicity
VD/VOD, from the English "Vacuum Degassing " and "Vacuum
Oxygen Decarburisation") respectively is a steel process
that has as its main objective that of producing steels
that meet high. quality standards and stainless steels.
[0004]The vacuum treatment makes it possible to achieve.
extremely low levels of sulphur, hydrogen and nitrogen,
improve the micro and macro purity of the steel, and, in
the case of VOD, decarbonise (reduce.the carbon content)
the steel.
[0005] Generally, VD/VOD systems are designed to operate
. round the clock and each treatment lasts from 35 to 120
minutes depending on the productivity required of the
plant and on operating practices.
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[0006] Although various plant solutions exist aimed at
meeting specific requirements (available installation
space, required productivity), normally a steel degassing
plant consists of the following components, as shown in
the general diagram in Figure 1:
[0007] -a vacuum chamber A, airtight to the outside, inside
which the ladle L containing the liquid steel is housed.
[0008] -a vacuum generator B, i.e. a system able to
aspirate gases until a pressure of less than 1 mbar
absolute is achieved inside the vacuum chamber.
[0009] -an intake line C which places the vacuum Chamber A
in communication with the vacuum generator B and the
latter with the stack D through which the gases generated
by the process are discharged;
[0010] -devices aimed at managing the process, installed .
along the intake line (valves illustrated below and gas
pressure and temperature measuring instruments, a heat
exchanger for cooling the process gas in output from the
vacuum chamber);
[0011]-a dust separation unit F generally composed of a
cyclone (to remove larger particles) and a filter (to
retain smaller particles);
[0012] -a gas insufflation system G, usually argon, in some
cases even nitrogen, for the agitation of the liquid
steel and removal of impurities within it;
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[0013] -an insufflation system E. of inert gas into the
intake duct in order to manage the foamy slag, raising
the pressure inside the isolated system;
[0014] -where the vacuum decarburisation of steel (VOD) is
also provided for, an oxygen injector controlled by an
auxiliary system is installed on the lid of the vacuum
chamber.
[0015] The vacuum chamber A consists of a lid Al and a tank
A2. Depending on which is the fixed part and which is the
mobile part there are two types of construction: a
"wheeled lid" when the tank is fixed and the lid mobile,
and a "wheeled tank" in the opposite case.
[0016] Generally speaking, depending on the operating
principle the vacuum generator B may be of two types:
with a steam ejector/liquid ring (technical solution more
popular in the past) or mechanical pumps (technology
becoming more widespread recently).
[0017] As shown in Figure 1, the devices for managing the
process and installed along the intake line C usually
comprise a valve V1 to return the vacuum chamber to
atmospheric pressure, a main valve V2 to isolate the
vacuum chamber from the vacuum generator, a valve V3 for
insufflating nitrogen to control the process.
[0018] The degassing plant in general is divided into two
parts by the main valve V2. There are thus two volumes: a
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tank volume and a retained volume.
[0019] The tank volume is returned to atmospheric pressure
after every vacuum treatment by opening the valve V1
which effectively places the vacuum chamber in
communication with the external environment. The retained
volume, instead, is generally kept in a vacuum thanks to
the main valve V2, which keeps it isolated from the
external environment. The maintenance of the vacuum in
the retained volume makes it possible to shorten the time
required to lower the pressure in the=system by using it
as a "plenum chamber" equalising the pressure between the
tank and retained volume at the moment of opening the
main valve V2. It should be noted that the tank is at
atmospheric pressure before opening the main valve V2.
[0020] Generally, a vacuum degassing process comprises the
following steps:
[00211- positioning the ladle containing liquid steel
inside the vacuum chamber and closing the lid;
[0022]- aspirating the gases contained inside the plant
' volume to achieve the required vacuum level (typically <
1 mbar);
[0023]- permanence at the operating pressure for the time
deemed appropriate (typically from 15 to 25 minutes) to
achieve the metallurgical objectives;
[0024]- restoration of the atmospheric pressure inside the
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vacuum chamber (opening of valve V1), and refining of the
chemical analysis by additions of materials in precise
quantities.
[0025]The intake gas is composed primarily of air up to a
pressure of about 100-150 mbar, then of metal vapours,
hydrogen and nitrogen coming from the steel. The suction
capacity of the vacuum generation system automatically
adjusts throughout the range of pressures. The operator
is requested to perform an adjustment only in the case of
abnormal chemical reactions inside the vacuum chamber
(especially in cases of foaming of the slag, present in
the ladle with the molten steel, to avoiding leakages of
incandescent material from the ladle itself).
[0026] The control of the entire process passes through the
movement of the lid and/or tank and the command of the
automatic cycles for the adjustment of the operating
conditions of the system (i.e. of the working points of
the vacuum generator in order to control the pressure
inside the vacuum chamber).
[0027] It is also known that during the entire degassing
process a large amount of dust is produced.
[0028]The material constituting the dust derives mainly
from the evaporation of metal elements present in the
liquid bath, subsequently condensed along the intake line
and the filter, from the reaction between the steel and
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the refractory and, to a lesser extent, from iron-alloys
and scorifiers.
[0029] During a VD process approximately 0.1-0.2 kg of dust
is produced per tonne of treated steel: during a complete
treatment up to 20-40 kg may be produced (considering for
example a ladle of a capacity of 200 tonnes of liquid
steel). A typical analysis of the dust composition
reveals a significant content of Zn, MgO, CaO, Pb, Mn.
[0030] In the VOD process ("Vacuum Oxygen Decarburisation",
a vacuum process with insufflation of oxygen to achieve
low levels of carbon in the liquid steel) the amount of
dust generated may reach 800-1000 kg (for 200 tonnes of
=
liquid steel).
[1:031] It is essential to have an effective dust collection
system to preserve the vacuum genelrator from wear or
clogging phenomena as well as to avoid dust emissions
into the atmosphere.
[0032]If pressure filtration is required, a cyclone
separator (tangential air intake) and a bag filter are
installed in series on the intake line. However, filter
installations also exist with an integrated cyclone.
[0033] Typically, the dust from these processes, because of
its composition, burns very easily in the presence of
oxygen. For this reason the bag filters (which are
currently the most common technology for such
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applications) require frequent and efficient cleaning,
which is typically done automatically after every
treatment, by blowing inert gas (nitrogen) in counter-
flow to the canvas bag, a technology known as "reverse
pulse jet".
[0034] Aside from environmental requirements concerning
atmospheric emissions, the need to install elements for
dust abatement (bag filters and cyclone) or not, is
determined by the degree of dust tolerated by the vacuum
system to be installed.
[0035] To date, there are two vacuum
generation
technologies based on completely different operating
principles: mechanical pumps and steam ejector systems.
Vacuum generation with mechanical pumps
[0036] In the terminology commonly used in the steel
industry, mechanical pump vacuum generation refers to a
vacuum generator which provides for the installation in
series of lobe type blowers (root pumps) and screw pumps
(screw pump) as illustrated in Fig.2. In this case the
screw pumps are also called "pre-vacuum pumps".
[0037] As a general principle, since each of these machines
performs a compression of the aspirated gas, a
compression "stage" is spOken of referring to one or more
machines operating in the same pressure range between
intake and discharge.
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[0038] The current most widespread plant solutions consist
of a series of a screw pump and at least two root pumps
in series, as shown in Figure 2.
[0039] The stages are conventionally named in ascending
numerical order (stage 1, ..., stage n) starting from
those closest to the vacuum chamber A. The last stage is
that which finally discharges the gases into the
atmosphere (pre-vacuum stage). Each stage may consist of
several pumps connected in parallel, as shown in Figure
3.
[0040] The criterion determining the arrangement in series
is as follows: screw pumps are capable of operating with
very high compression ratios (up to 1:1000) but with low
volumetric flow rates; root pumps instead are able to
dispose of large volumes of gas, but do not permit high
compression ratios (typically about 1:6).
[0041] In typical VD/VOD installations, operatively, the
screw pump alone is able to maintain a pressure of not
less than 20-50 mbar inside the vacuum chamber,
downloading the gases into the atmosphere. In order to
achieve a higher degree of depression (< lmbar) the
upstream installation of at least two stages of root
pumps is required. The latter, thanks to the type of
construction, (a double inner chamber alternately
liberated and obstructed by the rotating lobes) are most
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effective in moving very rarefied gases, which gases at
low pressures are.
[0042] In short, in stable operating conditions (i.e.
disregarding the initial evacuation transient ,of the
vacuum chamber starting from atmospheric pressure), the
early-stage root pumps aspirate the process gases at very
low pressures (< lmbar) and deliver them to the screw
pumps in the pressure range in which the latter operate
with higher compression effiCiency.
[0043] The main drawback of using mechanical pumps in a
configuration as described above is related to the need
to perform filtration of the aspirated gases in order to
retain the solid particulate which could block and/or
damage the rotating mechanical bodies (seizure) and
possibly contaminate the lubrication oil (contained in
the gear chamber in the case of deterioration of the
gaskets). The root pumps - while not meant for use in a
pulverulent environment - would theoretically be capable
of treating pulverulent gases without. running into
operational problems of seizure. In the long run however
oil contamination problems would arise. The biggest
problem relates to the screw pumps which would be forced
- without filtration - to treat the pulverulent gases
discharged by the root pumps, incurring in the aforesaid
seizure problems and leading to immediate blocking of the
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system.
[0044] In describing the typical operating conditions of a
vacuum generation system with mechanical pumps reference
is generally made to 4 automatic cycles which determine
the functioning of the main devices installed (valves,
filter, pumps):
[0045] - activation cycle of the system: the pumps are
started and the volume until the main valve ("retained
volume") is evacuated reaching a final pressure typically
< 5 mbar; the vacuum chamber at this stage remains at
atmospheric pressure and the pumps are kept at a minimum
rotation speed;
[0046] - Degassing cycle: the main valve opens to equalise
pressure in a controlled manner between the vacuum
chamber and the "retained" volume; the slow equalisation
is designed not to overstress the system from a
mechanical point of view (pumps and filterbags) and avoid
the instantaneous and violent oxidation of the pyrophoric
dust remaining on the surface of the filter bags after
the previous treatments. The pumps gradually speed up to
reach the maximum rotation speed. During the lowering of
the pressure the vacuum level in the system can be
controlled by slowing down/by-passing the pumps or =by
insufflating nitrogen. Typically the process pressure (<
lmbar) is reached in 6-8 minutes.
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[0047]- Stop vacuum cycle: when degassing is complete, the
main valve closes and the vacuum chamber is returned to
atmospheric pressure (permitting the subsequent opening
of the lid and addition of materials).
[0048]- Cleaning cycle: with the pumps isolated, the
filter bags are cleaned by means of a system of nitrogen
blows, the cleaning cycle being then followed by a dust
discharge cycle as necessary.
[0049] Once the cleaning cycle of the bags is compete, the
retained volume is again evacuated (up to pressures <
5mbar) preparing the system for the next degassing cycle.
[0050] The cleaning of the filter bags is a crucial aspect
for the performance of the mechanical pump system
because:
[0051] -an excessive accumulation of dust on the bags
increases the pressure losses through the filter,
limiting the minimum pressure which can be reached inside
the vacuum chamber;
[0052] -possible damage of the bags causes large quantities
of dust to reach the pumps. The operation of the system
may thus be jeopardised if the cleaning and maintenance
of filters is not properly conducted (correct setting of
the wash cycle with nitrogen, regular inspections of the
bags ...).
Vacuum generation with ejector pumps
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[0053]Ejector vacuum generators use as a propellent fluid
the superheated steam generated in a boiler or coming
from other sources. As a result of the acceleration of
the steam and the architecture of the ejector, the
process gas is aspirated and compressed.
[0054]Each ejector is sized to compress a given quantity
of gas, achieving a specific ratio between the intake and
discharge pressure (typically to the order of 1:5/1:15).
To operate between the pressure required by the process
(1. mbar) and atmospheric pressure (1000 mbar) several
different ejectors operating in series are therefore
required.
[0055] In this case too, in the arrangement in series, each
ejector is considered as a compression "stage". A stage
may however be composed of several ejectors in parallel
to increase the suction capacity of the system at higher
pressures (typically required during the evacuation phase
of the vacuum chamber).
[0056] Figure 4 shows a plant layout of a typical ejector
pumping station where Si, S2, S3 and S4 indicate ejector
stages, Cl, 02 and 03 inter-stage condensers and P a
collection tank or "hot pit". The S3 and S4 stages, in
this particular case, consist of pairs of A/B ejectors
operating in parallel. The activation sequence of the
individual stages is usually controlled by the pressure
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reached by the vacuum chamber, and is as follows (with
reference to Fig. 4): S4-S3-S2-S1.
[0057]To ensure maximum efficiency of the ejector system
(disposal of the maximum flow of process gas), heat
exchangers are installed in series with the ejectors to
condense the steam contained in the main gas flow.
[0058] The steam, in fact, acts only as a propeller = to
aspirate the process gases and condense as the pressure
increases and the temperature decreases.
[0059] The steam is thus made to condense inside the
"condensers" which are drained into a tank, called a "hot
pit".
[0060]It is clear that, in the absence of filtering
systems upstream of the ejector groups, the condensed
water has a higher concentration of dust, thus requiring
appropriate wastewater treatment plants and maintenance
operations for the disposal of the sludge channelled into
the "hot pit".
[0061] Fig. 4 shows a diagram of a typical ejector
consisting of four compression stages.
[0062] A variation of this diagram provides that the fourth
stage, or =alternatively a possible fifth stage, consists
of a liquid ring pump in place of an ejector. This
solution is generally preferred in systems with limited
steam availability or where required by plant or process
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requirements (limited space for installation, need to
operate stably at pressures above 100 mbar for VOD
systems).
[0063]The liquid ring pump is a mechanical, centrifugal-
type pump in which the compression of the gas, by means
of its confinement in a variable (gradually reduced)
volume, is consequential to the rotation of a liquid ring
generated by a centrifugal effect of a rotor, eccentric
to the casing (body) of said pump.
[0064] With the exception of the composition of the pumping
system and of the dust abatement group connected thereto,
the operation of an ejector/liquid ring plant passes
through operational sequences entirely similar to those
described for the mechanical pumps.
[0065]For the ejector systems it is not necessary, for the
purpose of protecting the pumping system, to abate the
dust to the extent of requiring the installation of a bag
filter since it lacks the geometric tolerances required
by the mechanical system, typical of root or screw pumps.
[0066] On the other hand, in some systems, to minimise
maintenance (cleaning of the ejectors and hot pit water
treatment) a cyclone or even a bag filter with related
automatic cleaning system may be installed.
[0067] Lastly, it is to be noted that in. the absence of
filter, elements, a large amount of dust is retained by
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the injected steam and by the water of possible liquid
ring pumps. The condensed steam between one ejector stage
and another helps to retain some of the dust generated
during the process. The condensed water is drained, as
mentioned above, into the "hot pit" (indicated. as P in
Fig. 4). Also the possible liquid ring in its contact
with the process gas helps to retain part of the residual
dust. It follows that in a liquid ring/ ejector system
the amount of dust contained in the gases discharged to
the stack is very low.
[0068]In conclusion, .the main difference as regards the
system layout between ejector systems and mechanical pump
systems lies in the presence of a bag filter (with all
the auxiliary elements for cleaning the bags and
discharging the dust), required in the latter case to
preserve the integrity of the machines.
[0069] The main limitation of injector vacuum generation
systems lies in their complexity and high plant and
running costs.
Presentation of the invention
[0070] Consequently, the purpose of the present invention
is to eliminate entirely or in part the drawbacks of the
prior art mentioned above, by providing a plant and
method for vacuum degassing liquid steel combining the
engineering/operational simplicity of a mechanical pump
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plant with the possibility to operate without filter
systems of an ejector plant.
[0071] A further purpose of the present invention is to
make available a plant for vacuum degassing liquid steel
which is operatively more reliable.
[0072] A further purpose of the present invention is to
make available a plant for _vacuum degassing liquid ,steel
which is cheaper to run.
[0073] A further purpose of the present invention is to
make available a plant for vacuum degassing liquid steel
which is at least comparable to conventional systems with
mechanical pumps, in terms of plant costs.
Brief description of the drawings
[0074] The technical characteristics of the invention,
according to the aforesaid purposes, can be seen clearly
from the contents of the following claims and the
advantages of the same will be more clearly
comprehensible from the detailed description below, made
with reference to the appended drawings, showing one or
more embodiments by way of non-limiting examples,
wherein:
[0075] -Figure 1 shows a general diagram of a steel
degassing plant;
[0076] -Figure 2 shows a general diagram of a conventional
vacuum generation system with mechanical pumps of the
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root and screw type;
[0077] -Figure 3 shows a general diagram of a conventional
vacuum generation system with mechanical pumps of the
root and screw type, with each stage composed of several
pumps in paralle1;,
[0078]-Figure 4 is a diagram of a conventional ejector
vacuum generation system;
[0079]-Figure 5 shows a general diagram of a liquid steel
vacuum degassing plant according to a preferred
embodiment of the present invention;
[0080] -Figure 6 shows a general diagram of a liquid steel
vacuum degassing plant according to an alternative
embodiment of the present invention;
[0081] -Figure 7 shows a general diagram of the vacuum
generation system in a liquid steel vacuum degassing
plant according to a preferred embodiment of the present
invention; and
[0082] -Figure 8 shows a general diagram of a liquid ring
pump.
Detailed description.
[0083]With reference to the appended drawings reference
numeral 1 globally denotes a plant for vacuum degassing
liquid steel according to the invention.
[0084] The plant 1 according to the invention can be used
for vacuum degassing with either the VD (Vacuum
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Degassing) technique or the VOD (Vacuum Oxygen
Decarburisation) technique and for all applications where
a vacuum treatment of liquid steel is required.
[0085] Here and henceforth in the description and the
claims, reference will be made to the vacuum degassing
plant of liquid steel 1 in conditions of use.
[0086] According to a general embodiment of the invention,
the plant for vacuum degassing liquid steel comprises:
[0087]- at least one vacuum chamber 2, suitable to
temporarily receive liquid steel inside it; and
[OW] - a vacuum generation system 10, connected to the
aforesaid at least one vacuum chamber 2 via an intake
duct 20.
[0089] The vacuum chamber 2 may be of any type suitable for
the purpose.
[0090] Preferably, the vacuum chamber 2 is configured so
that the liquid steel is brought inside via a ladle L,
but it may also be used directly to receive the liquid
steel.
[0091] In the first case, as shown in Figures 5 and 6, the
vacuum chamber 2 comprises a tank 3, which defines the
volume of the chamber 2' and is suitable to receive
therein the ladle L, and a lid 4 suitable to seal the
tank 3 tight when the ladle L is housed therein. The
vacuum chamber may be of the "wheeled lid" type when the
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tank is fixed and the lid mobile or of the "wheeled tank"
type in the opposite case.
[0092] Advantageously, as shown in Figures 5 and 6, the
vacuum chamber may be fitted with an insufflation system
30 of a washing gas, in some cases even nitrogen, for the
agitation of the liquid steel and removal of impurities
within it. In particular, this insufflation system 30 is
designed to feed one or more porous septums located on
the bottom of the ladle.
[0093] In the second case, according to an embodiment not
illustrated in the appended Figures, the vacuum chamber 2
may be configured to directly house within it the liquid
steel acording to an RH process. In this case, the liquid
steel is transferred temporarily from the ladle inside
the chamber. To such purpose the vacuum chamber is
connected to a ladle via two ducts: a delivery duct
through which the molten steel from the ladle is driven
by the difference in pressure inside thd vacuum chamber,
and a return duct, through which the treated molten steel
flows back from the vacuum chamber inside the ladle.
[0094] According to the invention, as shown in Figures 5
and 6, the vacuum generation system 10 comprises at least
two compression stages connected to each other in series,
of which:
[0095] -a first compression stage 11 works closer to the
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aforesaid at least one vacuum chamber 2 and consists of
one or more screw pumps 110; and
[0096] -a second compression stage 12 works farther away
from the aforesaid at least one vacuum chamber 2 to bring
the gas at least to atmospheric pressure and consists of
one or more liquid ring pumps 120.
[0097] The aforesaid one or more screw pumps 110 are sized
to be able to operate with compression ratios not
exceeding 1:12 if the discharge pressure is atmospheric,
and with compression ratios not exceeding 1:200 if the
discharge pressure is comprised between 50 and 120 mbar
absolute.
[0098] As will be specified below, the aforesaid one or
more screw pumps 110 are thus sized in a radically
different manner to conventional screw pumps.
[0099] Preferably the aforesaid one or more screw pumps 110
are sized to be able to operate with compression ratios
comprised between 1:3 and 1:10 if the discharge pressure
is atmospheric and, if the discharge pressure is between
50 and 120 mbar absolute, with compression ratios of
between 1:25 and 1:200, and preferably between 1:70 and
1:90.
[00100] Thanks to the fact of operating in the
aforesaid compression ratio ranges, the aforesaid one or
more screw pumps 110 are sized imposing internal
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tolerances (rotor/rotor and rotor/case), much higher than
those provided for in the conventional screw pumps used
as pre-vacuum stages as described above. This way, the
aforesaid screw pumps 110 are able to operate in direct
contact with pulverulent gases with high concentrations
of dust without contraindications for the moving
mechanical parts and thus without incurring in the
typical problems of screw pumps used as pre-vacuum stages
in conventional, mechanical degassing systems.
[00101] This is made possible by the fact that
according to the invention the screw pumps are used at
stages closer to the vacuum chamber and by the choice to
operate such pumps in the aforesaid compression ranges.
[00102] Operationally, the work of compressing the gas
is completed by the aforesaid one or more liquid ring
pumps which define the second compression stage (final),
further away from the vacuum chamber, exploiting the fact
that the liquid ring pumps are insensitive to dust.
[00103] Advantageously, the liquid ring pumps also
perform an important function of retaining the solid
particles dragged along by the main flow of the gases.
The pump service water is therefore used to trap the dust
generated by the degassing process .and then collect it in
a single point. This way the dust emission at a possible
discharge stack 40 is minimised, ensuring low
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environmental impact.
[0olm] Thanks
to the invention the vacuum generation
system 10 is thus able to aspirate directly from the
aforesaid at least one vacuum chamber 2 gases containing
dust in high concentrations, without the
contraindications typical of a conventional mechanical
pump system.
[00105]
Conventionally, contrary to the provisions of
the present invention, screw pumps are instead used in
the vacuum generation systems of degassing plants to
define the compression stages furthest away from the
vacuum chamber A. These pumps (pre-vacuum), despite
operating with compression ratios between 1:1 and 1:50,
and preferably between 1:2 and 1:40, are designed to work
with compression ratios up to 1:1000 with discharge at
atmospheric pressure. Conversely, the screw pumps 110
according to the invention are sized to operate at
maximum compression ratios of 1:12 with discharge at
atmospheric pressure. The traditional screw pumps must
therefore be built with very strict internal tolerances
(rotor/rotor and rotor/case). This makes them
particularly sensitive to the presence of dust in the
gases treated.
[00106] Thanks to the present invention, it is
therefore possible on the one hand to liberate the design
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of a liquid steel degassing plant from the installation
of a filtration device (usually a bag filter) required in
the case of mechanical pumps, and on the other to
drastically reduce the plant costs entailed by a
conventional steam ejector system.
[00107] Advantageously, the vacuum generation system 10
is sized to bring the vacuum chamber 2 to a degree of
vacuum between 0.2 and 5 mbar, and preferably between 0.5
and 1:5 mbar. As a result, the vacuum generation system
is sized to generate total compression ratios between
1:5,000 and 1:200.
[00108] As regards the sizing of the vacuum generation
system 10 according to the present invention, the
possible combinations in terms of number of screw pumps
110 and liquid ring pumps 120 are dictated by the design
choices from time to time made so as to minimise the
number of machines installed to get the level of
performance required by the process, i.e. evacuation
times of the vacuum chamber limited and degree of final
vacuum approximately < 1 mbar.
[00109] Advantageously, the vacuum generation system 10
may comprise one or more intermediate compression stages,
positioned in series between the first stage 11 and the
second stage 12 and each composed of one or more screw
pumps 110 having similar characteristics to those of the.
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first stage 11.
[00110] The term "similar characteristics" is taken to
mean that said one or more screw pumps of the
intermediate stages are sized to operate in the same
compression ranges as the screw pumps of the first
stages, thus making it possible to adopt internal
tolerances (rotor/rotor and rotor/case) much higher than
those provided for in conventional screw pumps. The size
of the screw pumps of the intermediate stages may be the
same or different to that of the screw pumps of the first
stages. The choice of size is dictated by the sizing of
the vacuum generation system.
[00111] One or more of the aforesaid compression stages
(first, second or intermediate) may each consist of two
or more pumps connected in parallel.
[00112] According to embodiments not shown in the
appended figures, the vacuum generation system may
consist of two or more parallel pumping modules, each of
which is composed at least of a first compression stage
11 with screw pumps and a second compression stage 12
with liquid ring pumps.
[00113] The total number of pumps installed per module
and the number of modules is defined in the design phase
with the objective of optimising the installation and
minimising the consumption of auxiliary elements (water,
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nitrogen, electricity).
[00114] Advantageously, modular configurations may be
adopted for the vacuum generation system 10, i.e.
separable into units installed in parallel, or "hybrid"
installations where the pumps are grouped on two stages
without modularity.
[00115] Preferably, the vacuum generation system 10 can
be isolated from the rest of the system by closing
appropriate isolation valves installed immediately
upstream of the pumps.
[00116] Preferably, as shown in Figures 5 and 6, the
intake duct 20 comprises a by-pass duct 21 able to
exclude from the gas flow the compressor stages formed of
the screw pumps 110. This solution can be adopted both in
the case of a modular structure, and a non-modular
structure.
[00117] Operatively, as will be resumed below, the
presence of the aforesaid by-pass 21 may be used to
exclude the screw pumps from functioning in some stages
of the degassing process.
[00118] Preferably, each of the screw pumps 110 used in
the degassing plant 1 according to the invention
comprises two screw rotors, kinematically synchronised
with each other via an electric axis.
[00119] For the connection and synchronization of the
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two screw rotors these pumps do not use the conventional
"mechanical axis", where an engine transmits movement to
a screw rotor while the other rotor is dragged/
synchronised by means of a series of gears in oil bath.
[00120] The term "electric axis" means the software
synchronisation of a pair of engines by means of an
inverter (one for each screw) and a pair of encoders. The
software instantly manages the parameters of the two
inverters so that the rotors are constantly synchronised.
Furthermore the two encoders control the angular
deviation of the axes of the screw rotors, so that these
are perfectly parallel to each other.
[00121] Operatively, any functional anomaly (e.g.
internal friction due to dust build-up) results in an
increase in torque and current absorption of the motors
(or of one of them) and, consequently, a possible
deviation in the angular speed of the rotors.
Advantageously, the software can act on the speed in real
time until equilibrium is restored, avoiding stresses and
overheating of the pump.
[00122] Compared to a solution with a mechanical axis,
this electric axis configuration does not require oil for
the lubrication of the gears. The absence of lubrication
oil is an advantage. In fact, due to the possible
difference in pressure between the compression chamber
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(lower pressure) and possible (concurrent) gear chambers
(higher pressure), the oil can be aspirated into the
process gas, mixing with the dust and generating
obstructions. Similarly, =certain operating phases,
dusty gases can inundate the gear chambers polluting the
oil.
[00123] The liquid ring pumps 120 used in the degassing
plant 1 according to the present invention are of the
type known per se and their operation is therefore well
known to a technician of the sector. A detailed
description of the same is therefore not provided but
merely reference to a number of concepts useful for
introducing some particular elements.
[00124] In particular, the liquid ring pumps used in
the present invention may have the structure shown in
Figure 8.
[00125] As shown in Figure 8, a liquid ring pump
compresses the process gas G' between an eccentric vane
rotor 121 and a ring 122 of water, called service water
W. Operatively, the dust carried by the process gas G'
necessarily comes into contact with the service water W
which acts as a collector. The pump 120 ejects the
compressed gas G" together with a minimum amount of
pulverulent service water. The mixture G " + W of gas and
pulverulent water reaches a separator 123 which separates
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the gas (now at atmospheric pressure and directed to the
stack) from the "dirty "water which is collected in the
lower part of the separator 123. Advantageously, a
replenishment 124 of, the water W is provided to offset
the losses from evaporation.
[00126] More specifically, the service water W can be
handled in two ways: in an open circuit or closed
circuit.
[00127] With closed-circuit management the water W is
recirculated until the saturation limit of dust, at which
the pump performance drops. At thisT point all the service
water W is discharged and replaced with clean water.
[00128] With open circuit management, the water is
continuously discharged from the separator (through the
opening 125 illustrated in Figure 8), while a line of
clean water 124 continuously tops up the service circuit
of the liquid ring pump.
[00129] Advantageously, the resulting water contains
dust which is now inert and can be handled in two
different ways.
[00130] According to a first method, the pulverulent
water is collected in a decanting bath with an overflow
which leads to a second bath. From here the pulverulent
water is sent on to a water treatment plant, by .means of
centrifugal pumps, and treated therein in the
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conventional way.
[00131] According to a second method, as shown
schematically in Figure 7, the pulverulent water leaving
the separator can be filtered on site using known
methods.
[00132] Advantageously, as shown in Figure 7, the plant
1 comprises an auxiliary unit 50, which, in addition to
replenishing the water dispersed by the liquid ring pumps
in the process gases, separates the dust contained in the
water and recirculates it to the pump.
[00133] A continuous cycle operation guarantees both
the controlled removal of the dust (avoiding internal
build-up) and optimal operation of the liquid ring pump
120 thanks to the cooling and cleaning of the top-up
water.
[00134] The auxiliary unit 50 may be centralised or
located on board of each liquid ring pump or module,
maintaining however the same functions.
[00135] Alternatively to the aforesaid auxiliary unit
50, the plant 1 may comprise at least one continuous
replacement device of the service water used by the
liquid ring pump, without recirculation, with non-
returnable water.
[00136] Advantageously, as shown in Figures 5 and 6, in
the section comprised between the vacuum chamber 2 and
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the vacuum generation system 10 the intake duct 20
comprises a connection branch 28 to the atmosphere
equipped with a first control valve 23. This first
control valve 23 is opened at the end of the degassing
process to return the vacuum chamber 2 to atmospheric
pressure before taking out the treated liquid steel.
[00137] Advantageously, as shown in Figures 5 and 6, in
the section comprised between the vacuum chamber 2 and
the vacuum generation system 10 the intake duct 20 may
comprise a connection branch 29 to a tank (not shown)
containing inert gas (nitrogen or argon), equipped with a
second control valve 24. The inert gas can be insufflated
by opening the second valve 24 in order to manage the
foamy slag, raising the internal pressure.
[00138] According to a preferred embodiment illustrated
in ,Figure 5, the degassing plant 1 does not comprise a
filtration device of the gases, which leave the vacuum
chamber 2 and have to pass through the vacuum generation
system 10. Regardless of the concentration level of the
dust in said gases, the gases in output from the vacuum
chamber 2 are aspirated directly by the vacuum generation
system without a preventive gas filtration step. As noted
previously, this is possible thanks to the present
invention.
[00139] According to an alternative embodiment
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illustrated in Figure 6 the degassing plant 1 may
comprise at least one filtration device 25 of the gases
leaving the vacuum chamber 2 and passing through the
vacuum generation system 10. Such a filtration device 25
is arranged between the vacuum chamber 2 and the vacuum
generation system 10.
[00140] Operatively, the gases exiting the vacuum
chamber 2, before being aspirated by the vacuum
generation system, are subjected to filtration in order
to abate at least partially the dust content present in
the gases. Thanks to the present invention, the abatement
of the dust may be partial and bland, given that the
possible presence of dust does not affect the operation
of the vacuum generation system 10. The preventive
filtration step may be provided so as to optimise the
management of dust in the system, reducing the load of
dust to be managed by means of the liquid ring pumps.
[00141] The aforesaid filtration device 25 may consist
of a bag filter, a cyclone or of an integrated bag filter
and cyclone system.
[00142] In particular, according to the alternative
embodiment illustrated in Figure 6, the plant 1 comprises
at least an isolation valve 22 which is installed on the
intake duct 20 between the vacuum chamber 2 and the
filtration device 25. Such isolation valve 22 is placed
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downstream of the branching point of the intake duct 20
into the aforesaid connection branch 28 to the
atmosphere. The isolation valve 22 divides the plant 1
into two parts, thus identifying two volumes. A first
part comprises the vacuum chamber (tank volume); the
second part comprises the filtration device and the
vacuum generation system (retained volume).
[00143] Operatively, the tank volume is returned to
atmospheric pressure after every vacuum treatment by
opening the aforesaid first control valve 23 which places
the vacuum chamber in communication with the external
environMent. The retained volume may, instead, be always
kept in a vacuum thanks to the isolation valve 22 which
effectively keeps it airtight. The maintenance of the
vacuum of the retained volume makes it possible to
shorten the time required to lower the pressure in the
system by using it as a "plenum chamber" equalising the
pressure between the tank and retained volume at the
moment of opening the isolation valve 22.
[00144] The presence of the isolation valve 22. is
preferred in the case in which the plant 1 is equipped
with a filtration device 25 (in particular if it is a bag
filter) as shown in Figure 6. In this case, the retained
volume is very high due to the presence of the filtration
device.
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[00145] Advantageously, in the case in which the plant
1 is not equipped with a filtration device 25 (see Figure
5), the isolation valve 22 need not be installed, since,
in the absence of the filtration device, the retained
volume is reduced and therefore the advantages associated
with maintaining said volume in a vacuum are limited.
[00146] Advantageously, in the case in which the plant
1 is used for the vacuum degassing with VOD (Vacuum
Oxygen Decarburisation) technique it may comprise a heat
exchanger (not shown in the appended figures) for cooling
the process gases. In fact, with the VOD technique, as a
result of the injection of oxygen and consequent
decarburisation of the steel, the temperatures involved
increase significantly. The heat exchanger should be
placed upstream of the possible filtration device 22 and
downstream of the possible isolation valve (if present),
in the second part of the system (retained volume).
* * *
[00147] The present invention also relates to a method
for vacuum degassing liquid steel.
[00148] In particular, the method according to the
invention may be implemented in a degassing system
according to the invention, in particular as described
above. The parts in common with the plant 1 described
above have been indicated using the same alpha-numerical
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references.
[00149] According to a general embodiment of the
invention, the method for vacuum degassing liquid steel
comprises the following operating steps:
[00150] a) providing at least one vacuum chamber 2
suitable to temporarily receive liquid steel inside it;
[00151] b) placing liquid steel in said vacuum chamber
2;
[00152] c) evacuating the vacuum chamber 2 through a
vacuum generation system 10 creating in said chamber a
predefined degree of vacuum and maintaining it for a
predetermined period of time so as to complete the
operation of degassing the liqufd steel; and
[00153] d) bringing again the vacuum chamber 2 to
atmospheric pressure and withdrawing the degassed liquid
steel.
[00154] . According to the invention, the vacuum
evacuation step c) is conducted by means of a vacuum
generation system 10 comprising at least two compression
stages connected together in series, of which:
[00155] -a first compression stage 11 works closer to .
the aforesaid at least one vacuum chamber 2 and consists
of one or more screw pumps 110; and
[00156] -a second compression stage 12 works farther
away from the aforesaid at least one vacuum chamber 2 to
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bring the gas at least to atmospheric pressure and
consists of one or more liquid ring pumps 120.
[00157] The aforesaid one or more screw pumps 110 are
sized to be able to operate with compression ratios not
exceeding 1:12, if the discharge pressure is atmospheric,
and with compression ratios not exceeding 1:200, if the
discharge pressure is comprised between 50 and 120 mbar
absolute.
[00158] Preferably, the aforesaid one or more screw
pumps 110 are sized to be able to operate with
compression ratios comprised between 1:3 and 1:10 if the
discharge pressure is atmospheric, and, if the discharge
pressure is between 50 and 120 mbar absolute, with
compression ratios of between 1:25 and 1:200, and
preferably between 1:70 and 1:90.
[00159] Preferably, in the aforesaid evacuation step
c), the vacuum chamber 2 is brought to work at a degree
of vacuum between 0.2 and 5 mbar, and preferably between
0.5 and 1.5 mbar.
[00160] According to a preferred embodiment of the
method, the evacuation step c) provides for the direct
aspiration of the gases from the vacuum chamber 2 through
the aforesaid vacuum generation system 10 without a
preventive filtration step of the gases, independently of
the level of dust concentration in the gases themselves.
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[00161] According to an alternative embodiment of the
method, the evacuation step c) provides for the
aspiration of the gases from the vacuum chamber 2 through
the aforesaid vacuum =generation system 10 with a
preventive filtration step of the gases, to reduce the
dust concentration in said gases before their passage
through the vacuum generation system 10.
[00162] Preferably, the evacuation step c) comprises:
[00163] - an initial evacuation step cl) wherein the
vacuum chamber 2 is brought from atmospheric pressure up
to about 300 mbar using only the liquid ring pumps of the
vacuum generation system 10; and
[00164] - a final evacuation step c2), wherein the
vacuum chamber 2 is brought from the pressure of about
300 mbar to the predefined degree of vacuum, also using
the screw pumps.
[00165] This operating mode makes it possible to
minimise the amount of dust which the screw pumps must
handle, to the benefit of the operation of such pumps.
This operating mode takes advantage of the presence of
the by-pass 21 which is present on the intake duct and
which permits the exclusion of the screw pumps from the
passage of the gases.
[00166] Advantageously, during the evacuation step c)
the suction capacity of the vacuum generation system 10
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can be varied to reduce any phenomena of foaming of the
slag in the liquid steel. The suction capacity is varied
by=slowing or excluding one or more of the pumps of the
vacuum generation system 10, preferably the liquid ring
=
pumps 120.
[00167] Preferably, the above change in suction
capacity is carried out when the internal pressure of the
vacuum chamber 2 is between 300 mbar and 1 mbar, i.e.
during the final evacuation step c2).
[00168] Preferably, the method according to the
invention comprises a=step f) of treating the service
water used by the aforesaid one or more liquid ring pumps
120. This treatment step f) is carried out preferably
during the evacuation step c). The treatment consists of
filtering the dust from the water or of continuously
replacing the water.
[00169] Advantageously, the method comprises a step e)
of mixing the molten steel at least during the evacuation
step c), in particular by insufflating inert gases into
said steel.
[00170] The invention permits numerous advantages to be
achieved, in part already described.
[00171] The plant 1 for vacuum degassing liquid steel
according to the invention combines the plant simplicity
of a mechanical pump plant with the possibility to
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operate without the filter systems of an ejector plant.
[00172] Thanks to the invention it, is therefore
possible to minimise the equipment installed in a
degassing plant. .
[00173] This way it ensures greater flexibility in the
design phase (layout and auxiliaries) as well as allowing
operation of the system while minimising maintenance
costs and possible repairs of the parts most subject to
wear in a conventional system. In particular, it reduces
periodic inspections and eliminates the need to replace
the filter bags.
[00174] The plant 1 for vacuum degassing liquid steel
according to the invention is therefore:
[00175] -operatively more reliable; and
[00176] -cheaper to run.
[00177] In terms of plant costs, the plant 1 for vacuum
degassing liquid steel is at least comparable to
conventional systems with mechanical pumps and certainly
less expensive than conventional systems with ejectors.
[00178] The advantages set forth above for the plant 1
according to the invention extend to the degassing method
according to the invention.
[00179] The invention thus conceived thereby achieves
the intended objectives.
[00180] Obviously, its practical embodiments may assume
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forms and configurations different from those described
while remaining within the sphere of protection of the
invent ion.
[00181] Furthermore, all the details may be replaced by
technically equivalent elements and the dimensions,
shapes and materials used may be any as needed.
